Just to reply to title of the post, I'm happy with the Linux kernel, basically. There isn't a problem with its licensing or anything, so I don't have any motivation to experiment with HURD (though I absolutely understand people who would do that for the fun of tinkering, as a sometime Plan9 user). Microkernels were a research topic in the 90s but at this stage the design space has been explored and they have been shown to be much more complex and difficult to pull off than people realised at first, mainly because of all the inter-process communication required. Torvalds was very outspoken on this, insisting that code that needs to be very fast needs to run in the same address space, and he was right as it turned out. The other side of the argument is about security, but Linux shows that monolithic kernels can be made secure if the engineering is good enough. The fact that Torvalds just got on with it in the tried and trusted monolithic style is a classic case of Worse is Better.